Liquid crystalline side chain polymer electrolytes
Ion transport in polymer electrolytes is, in general, strongly coupled to the segmental motions of the host polymer. Thus, ionic conductivity is not usually observed below the glass transition. Recent research has highlighted examples of polymer electrolytes in which the ionic conductivity is not coupled to the polymeric motions. This thesis investigates one such group of polymers: namely, liquid-crystalline side-chain polymers. The aim is to investigate the effect of different liquid crystal morphologies and to gain insight into the ion transport mechanism. The thermal, mechanical and electrical properties of three different liquid-crystalline side-chain polymer electrolytes have been investigated. These included calamitic and discotic phases, with clearing temperatures in the range 53-95°C. This study shows a general behaviour characteristics of a range of such systems. After passing through the clearing transition into the liquid crystalline phase, the conductivity falls away more slowly as the glass transition is approached. Conductivities at the clearing temperatures are typically 10-7 S cm-1. The conductivities of selected complexes are examined under variable pressure (0-160 MPa), and activation volumes, typically in the range 20-40 cm3 mol-1, are obtained. Plots of EA versus VA reveal that the conductivity behaviour deviates from the PEO mechanism to varying degrees. Generally, introducing liquid crystalline side chains introduces more free volume into the system, and hence lowers the activation volumes. This new result supports an earlier proposal that the liquid crystalline phase opens up and stabilises the ethylene oxide structure, trapping in free volume, which is retained in the system as the glass transition is approached. Ion transport, however, remains dependent on local motions of the polymer. Optimisation of electrolyte properties depends on careful choice of system parameters (salt concentration, polymer morphology, etc).